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Meanwhile, Justin, 31, also appeared to be relaxing as he went for a dip in the ocean.

It
was reported in January this year that the pair had started seeing each
other again, after they were spotted at a hockey game in Philadelphia.

No blurred lines here: Kate showed off her tan lines in her skimpy bikini, as she holidayed with her on-again boyfriend Justin

Hanging out: The pair were seen relaxing together side by side on two sun loungers in the sun

The pair first started dating in early January 2013, but split up soon after.

Last
month, Kate also celebrated her 22nd birthday with Justin, quashing
rumours that she was in a relationship with Dancing With the Stars'
Makism Chmerkovskiy.

The
blonde beauty had lunch with the Detroit Tigers pitcher at Tavern on
Rush in Chicago, as he had a game there on the same night.

Back together: Justin - the baseball pitcher for the Detroit Tigers of Major League Baseball - looked relaxed as he sat beside his girlfriend on holiday

Careful there! Kate almost falls out of her barely there bikini as she lays on a sun lounger

Taking a break: The busy 22-year-old model is currently taking time out of her hectic schedule

Holiday read: Kate took a break from her book as she looked up lovingly at her her beau Justin with lounging in Mexico

An eyewitness told E! News: 'They seemed very happy.'

Later that evening, Kate joined five female friends at the Tiki bar, Three Dots and a Dash, for another party.

Another source said: 'She looked stunning in a red dress. They toasted with champagne at midnight.'

The
bikini model sparked speculation she had reunited with 34-year-old
Maksim, whom she split from in December, after he posted a photograph of
them cuddling on Instagram on May 31 and deleted the snap a few minutes
later.

Lap of luxury: Justin appeared to be in holiday mode as he took a dip in Mexico on Tuesday

Rekindled: The pair first started dating in early January 2013, but split up soon after

Stunner: At one point, the busty Sports Illustrated model seemed to barely contain her curves as she laid down on a sun lounger

The
image showed Kate smiling widely with her arm wrapped around him and
was taken while they was partying at Tao nightclub in New York City.

Sources
close to the former couple previously blamed their respective hectic
schedules for their break-up, and Maksim recently flew to India to be a
guest-judge on the local version of the reality dancing competition.

It is no surprise that Kate looked fantastic while relaxing on the beach, as the star recently confessed she sometimes spends 'three whole days' on her appearance.